Systems and methods for forming selective metal electrode layers for resistive switching memories

ABSTRACT

A method for selectively depositing a platinum layer on a substrate includes providing an electroless deposition solution including a platinum precursor and at least one of water and/or a pH balancing solution. A substrate including a patterned metal layer and one or more dielectric layers is immersed in the electroless deposition solution for a first predetermined period. The platinum layer is selectively deposited on the patterned metal layer but not on the one or more dielectric layers. The substrate is removed from the electroless deposition solution after the first predetermined period.

FIELD

The present disclosure relates to systems and methods for forming selective metal electrode layers for resistive switching memories.

BACKGROUND

The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

Electronic devices use integrated circuits including memory to store data. Currently, there is a significant demand in the marketplace to increase memory density in electronic devices. Until recently, the increased demand has been met in part by reducing minimum feature sizes of NAND flash memory (currently below 20 nm) and developing 3D NAND memories. However, patterning costs of NAND memory using these approaches have risen steeply.

While NAND memories are approaching the scaling limit, resistive random access memory (RRAM) can be scaled down to 2 nm. In addition to increased ability to scale, RRAM also has faster switching speeds and longer data retention time as compared to NAND flash memory. In RRAM, a platinum (Pt) layer is typically used as a bottom electrode. One challenge in fabricating RRAM devices involves patterning of the Pt layer. In a typical patterning process, a material is deposited onto the substrate as a film layer. A photolithography process writes specifically patterned polymer or photoresist layers above the film layer. Exposed portions of the film layer (film not covered by the photoresist) are subsequently etched to match the pre-defined pattern of the photoresist layer. However, this approach does not work well for Pt. The Pt layer is hard to dry etch since Pt does not form volatile products with common etchants. Other approaches such as wet etching are unable to create high-aspect-ratio (HAR) features that are required in most devices. Still other approaches such as sputter etching are detrimental to the substrate and the entire device stack.

Patterning may also be accomplished by selectively depositing or growing a film layer on substrates in pre-defined patterns. Selectivity means that the film layer is deposited on some types of materials but not others. However, both physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes are not sufficiently selective. In other words, the film layer is also deposited on “non-growth” regions of the substrate when using PVD and CVD. Although atomic layer deposition (ALD) can potentially guarantee sufficient selectivity, the deposition rate of ALD is generally too slow, which increases cost. In addition, the film layer may be mixed with impurities from ligands in the ALD precursors.

SUMMARY

A method for selectively depositing a platinum layer on a substrate includes providing an electroless deposition solution including a platinum precursor and at least one of water and/or a pH balancing solution. A substrate including a patterned metal layer and one or more dielectric layers is immersed in the electroless deposition solution for a first predetermined period. The platinum layer is selectively deposited on the patterned metal layer but not on the one or more dielectric layers. The substrate is removed from the electroless deposition solution after the first predetermined period.

In other features, the method include adding a reducing agent to the electroless deposition solution after the substrate is immersed in the electroless deposition solution.

In other features, the reducing agent comprises titanium chloride. The reducing agent comprises hydrazine.

In other features, the patterned metal layer includes at least one of cobalt (Co), ruthenium (Ru), tungsten (W), tungsten nitride (WN), and/or titanium nitride (TiN).

In other features, the pH balancing solution includes ammonium hydroxide.

In other features, the pH balancing solution adjusts the pH of the electroless deposition solution to a range between 8 and 11. The pH balancing solution adjusts the pH of the electroless deposition solution to a range between 9 and 10. In other features, the substrate is pre-cleaned to remove an oxide layer on the patterned metal layer prior to depositing the platinum layer.

In other features, the method includes depositing a palladium activation/seed layer prior to depositing the platinum layer.

In other features, the substrate forms part of a resistive random access memory (RRAM).

In other features, the method includes depositing a first layer on the platinum layer and depositing a second layer on the first layer. The first layer comprises at least one of HfO_(x), TiO_(x), TaO_(x), WO_(x), Al₂O₃, or combinations thereof. The second layer includes titanium nitride (TiN).

Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is a side cross-sectional view of an example of a substrate;

FIGS. 2A and 2B are side cross-sectional views of examples of substrates after platinum layers are deposited according to the present disclosure;

FIGS. 3A and 3B are side cross-sectional views of examples of the substrates in FIGS. 2A and 2B, respectively, after additional layers are deposited;

FIG. 4 illustrates an example of a method for depositing the platinum layer according to the present disclosure.

In the drawings, reference numbers may be reused to identify similar and/or identical elements.

DETAILED DESCRIPTION

Electroless deposition according to the present disclosure is used to perform selective deposition of a platinum layer to a desired thickness on pre-patterned substrates. The electroless deposition process according to the present disclosure enables further fabrication of RRAM stacks. In one example, a substrate with a patterned metal layer and a dielectric layer is immersed in an electroless deposition solution including a Pt precursor. In some examples, the electroless deposition solution further includes a reducing agent.

Pt is selectively deposited only on the patterned metal layer and not on the dielectric layer. In some examples, the dielectric layer may include SiO₂. In some examples, the patterned metal layer includes cobalt (Co), ruthenium (Ru), tungsten (W), tungsten nitride (WN), or titanium nitride (TiN). Pt does not nucleate on the dielectric layers in an appreciable manner and therefore deposition does not occur on the dielectric layers. In contrast, Pt nucleation on the patterned metal layer occurs relatively quickly and forms a Pt layer that is thin and relatively uniform.

Electroless deposition is also able to grow the Pt layer much faster than ALD. In some examples, the electroless deposition process can finish in about 15 minutes. The film thickness may be tuned by altering the duration of the deposition period and/or altering the chemistry of the reaction mixture.

Referring now to FIG. 1-3B, a portion of an example of an RRAM stack is shown. In FIG. 1, a substrate 100 includes a metal layer 120 that has been patterned by a dielectric material 122-1 and 122-2 (collectively dielectric material 122). The patterned metal layer 120 may be deposited on an underlying material (not shown) using physical vapor deposition (PVD), atomic layer deposition (ALD), chemical vapor deposition (CVD) or any other suitable process.

In FIG. 2A, a Pt layer 124 is deposited on the patterned metal layer 120 using electroless deposition as described herein. In FIG. 2B, an activation/seed layer 126 is deposited before the Pt layer 124. The activation/seed layer 126 may be used when Pt is hard to nucleate on the selected patterned metal layer 120. In some examples, the activation/seed layer 126 includes a palladium (Pd) activation/seed layer, although other materials may be used.

In FIGS. 3A and 3B, one or more additional layers such as layers 128 and 132 may be deposited on the patterned metal layer 120. In some examples, the layer 128 includes metal oxides such as hafnium oxide (HfO_(x)), titanium oxide (TiO_(x)), tantalum oxide (TaO_(x)), tungsten oxide (WO_(x)), aluminum oxide (Al₂O₃), or any combination of these materials, although other materials may be used. The oxides including HfO_(x), TiO_(x), TaO_(x), and WO_(x) can be stoichiometric or non-stoichiometric (with oxygen vacancies). In some examples, the layer 132 includes titanium nitride (TiN), although other materials may be used.

Referring now to FIG. 4, a method 210 for depositing Pt on a patterned metal layer is shown. In some examples, the steps are optionally performed in a glove box, although other processing environments may be used. At 220, the substrate is optionally pre-cleaned for a first predetermined period before the electroless deposition process. The cleaning procedure will vary depending upon the chemical properties of the substrate. In some examples, the cleaning process involves removal of the native oxide layer at a surface of the patterned metal layer. For example, if the patterned metal layer includes Ru, the substrate may be pre-cleaned with boron hydride to remove native RuO_(x), although other cleaning solutions may be used. A similar approach may be used for metal layers including other types of metals.

At 224, control determines whether the first predetermined period is up. For some substrates on which Pt is hard to grow, a palladium (Pd) activation/seed layer can optionally be deposited at 225. Pt is deposited smoothly when these substrates are covered with the Pd activation/seed layer. At 228, the substrate is immersed in a electroless deposition solution including platinum precursor.

For example only, the patterned metal layer may comprise Co or Ru and the Pt electroless deposition solution may comprise chloroplatinic acid (H₂PtCl₆). For example only, the concentration of the Pt precursor relative to water may be approximately 8 wt %, although other precursor wt % may be used depending upon the volume of electroless deposition solution. Other Pt precursors including Pt (IV) salts such as K₂PtCl₆ may be used.

In some examples, the optional annealing that may occur later in processing of the stack may be limited to a predetermined temperature. For example, the optional annealing may be limited to approximately 200° C., 250° C., or 300° C. when the patterned metal layer includes Co to limit intermixing with the Pt layer. For example, the optional annealing may be limited to approximately 300° C., 350° C., or 400° C. when the patterned metal layer includes Ru to limit intermixing with the Pt layer.

The electroless deposition solution may also include water and/or a pH balancing solution. In some examples, the pH balancing solution may include ammonium hydroxide (NH₄OH). In some examples, the pH balancing solution adjusts the pH of the electroless deposition solution to a range between 8 and 11. In other examples, the pH balancing solution adjusts the pH of the electroless deposition solution to a range between 9 and 10. Lower pH values such as 4 may be associated with lower quality deposition of Pt film.

The substrate is immersed in the electroless deposition solution for a second predetermined period to allow selective electroless deposition of the Pt film to proceed to a desired film thickness. At 232, after the substrate is immersed in the electroless deposition solution, a reducing agent is optionally added to the electroless deposition solution. In some examples, the reducing agent is added immediately after the substrate is immersed in the electroless deposition solution. For example, the reducing agent may include titanium (III) chloride (TiCl₃) in an HCl solution. Alternately, hydrazine may be used as an alternative reducing agent.

At 236, control determines whether the second predetermined period is up. The second predetermined period is selected to allow the selective electroless deposition of the Pt film to proceed to a desired film thickness. After the second predetermined period at 240, the substrate is removed from the electroless deposition solution. In some examples, annealing may optionally be performed at 242.

Additional details relating to electroless deposition may be found in commonly assigned U.S. patent application Ser. No. 14/243,793, filed on Apr. 2, 2014 and entitled, “Electroless Deposition of Continuous Platinum Layer Using Complexed C_(o) ^(2t) Metal Ion Reducing Agent”, which is hereby incorporated by reference in its entirety.

In one example, a substrate includes a patterned cobalt (Co) layer that is deposited using PVD. The substrate is dipped into a citric acid solution for a predetermined period such as 2 min to perform a pre-clean step and to remove a native oxide layer.

60 mL of water, 7.5 mL of pH balancing solution, and 2.25 mL of 8 wt % H₂PtCl₆ solution are mixed in a beaker. After pre-cleaning is complete, the substrate is inserted into the beaker. 5.5 mL of TiCl₃ solution is added immediately. The pH of the reaction mixture (after adding TiCl₃) is usually about 9.4. The pH balancing solution may include 11.5 mL methoxyacetic acid, 98 mL 29% ammonium hydroxide, and 90.5 mL water.

Electroless deposition according to the present disclosure allows efficient patterning of Pt at lower cost than other methods. Eliminating the Pt etch step reduces cost by eliminating the use of etchants and the photolithography step before etching. The usage of Pt (or Pt precursors) can also be minimized. Electroless deposition according to the present disclosure is also highly selective.

The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.” It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. 

What is claimed is:
 1. A method for selectively depositing a platinum layer on a substrate, comprising: providing an electroless deposition solution including a platinum precursor and at least one of water and/or a pH balancing solution; immersing a substrate including a patterned metal layer and one or more dielectric layers in the electroless deposition solution for a first predetermined period, wherein the platinum layer is selectively deposited on the patterned metal layer but not on the one or more dielectric layers; and removing the substrate from the electroless deposition solution after the first predetermined period.
 2. The method of claim 1, further comprising adding a reducing agent to the electroless deposition solution after the substrate is immersed in the electroless deposition solution.
 3. The method of claim 2, wherein the reducing agent comprises titanium chloride.
 4. The method of claim 2, wherein the reducing agent comprises hydrazine.
 5. The method of claim 1, wherein the patterned metal layer includes at least one of cobalt (Co), ruthenium (Ru), tungsten (W), tungsten nitride (WN), and/or titanium nitride (TiN).
 6. The method of claim 1, wherein the pH balancing solution includes ammonium hydroxide.
 7. The method of claim 6, wherein the pH balancing solution adjusts the pH of the electroless deposition solution to a range between 8 and
 11. 8. The method of claim 6, wherein the pH balancing solution adjusts the pH of the electroless deposition solution to a range between 9 and
 10. 9. The method of claim 1, further comprising pre-cleaning the substrate to remove an oxide layer on the patterned metal layer prior to depositing the platinum layer.
 10. The method of claim 1, further comprising depositing a palladium activation/seed layer prior to depositing the platinum layer.
 11. The method of claim 1, wherein the substrate forms part of a resistive random access memory (RRAM).
 12. The method of claim 1, further comprising: depositing a first layer on the platinum layer; and depositing a second layer on the first layer.
 13. The method of claim 12, wherein the first layer comprises at least one of HfO_(x), TiO_(x), TaO_(x), WO_(x), Al₂O₃, or combinations thereof.
 14. The method of claim 13, wherein the at least one of HfO_(x), TiO_(x), TaO_(x), and/or WO_(x) are stoichiometric.
 15. The method of claim 13, wherein the at least one of HfO_(x), TiO_(x), TaO_(x), and/or WO_(x) are non-stoichiometric.
 16. The method of claim 12, wherein the second layer includes titanium nitride (TiN). 